Cathode sputtering processes are widely used for depositing thin coatings or films of material onto substrates. Such a process takes place in an evacuated chamber containing a small quantity of an ionisable gas, for example argon; electrons emitted from a source held within the chamber ionise the gas to form a plasma and the target comprising the material to be sputtered is bombarded by the ions causing atoms of the target material to be dislodged and subsequently deposited as a film on the substrate being coated.
It is well known that in magnetron apparatus the rate of deposition may be increased by the use of magnetic means, for example an array of permanent magnets positioned in a predetermined manner, (commonly as a closed loop), associated with the cathode target to define the area or region, often referred to as the "race-track", from which sputtering, or erosion of the target, occurs.
The target is commonly of flat, elongate and rectangular form and the substrate being coated is moved continuously or intermittently relative to the target during the sputtering process. Apparatus having such a target and having magnetic means of the type described above is known as a planar magnetron.
One disadvantage of the conventional planar magnetron is that the race track along which the sputtering takes place is relatively small and this causes erosion of the target surface in a relatively small, for example ring-shaped, region corresponding to the shape of the closed loop magnetic field. Thus, only a small portion of the total target surface area is consumed before the target needs replacing.
It is also known, however, to replace the planar target with a hollow cylindrical one comprising, or having the outer surface thereof coated with, the material to be sputtered. Apparatus containing such a target is known as a cylindrical magnetron and is sold by Airco Coating Technology (a division of the Applicants) under the trade mark "C-Mag".
In cylindrical magnetron apparatus, the cylindrical target is rotatable about its longitudinal axis either continuously or intermittently during use so that sputtering is not confined to one particular area of the target surface. As with the planar magnetron, the substrate being coated is moved, again either continuously or intermittently, relative to the target in a direction transverse to the longitudinal axis of the target.
The magnetron means in the form of an array of permanent magnets are placed within the cylindrical target in a position closest to the substrate during coating and are generally held stationary within the cylindrical target whilst (in use) the latter rotates. However, some adjustment of the magnet array relative to the target is normally possible for overall adjustment prior to each sputtering process. Such apparatus therefore allows a much greater amount of the target to be consumed in the sputtering process by selectively bringing different portions of the target into a sputtering position in respect of the magnetic field formed by the magnets.
In reactive sputtering, a "reactive" gas is introduced into the evacuated chamber in addition to the normal ionisable gas (commonly argon). This reactive gas is present to react in situ with particles of the target material to produce a sputtered coating incorporating the reaction products. For example it is known to deposit a coating of a mixture of silicon and aluminium oxides form a silicon-aluminium alloy target by sputtering in a chamber containing argon and oxygen. Alternatively replacement of the oxygen by nitrogen as the reactive gas, can produce a coating comprising a mixture of silicon and aluminium nitrides or replacement by a hydrocarbon can produce a coating comprising a mixture of carbides.
However, in such sputtering systems, there can be a tendency for the reactive gas to react with the target material prior to the individual target material particles being sputtered from the target, thereby forming a layer in the target of oxide, nitride or carbide (or whatever). The formation of such layer on the target is clearly undesirable as it affects sputtering performance.
It is common practice for the ionisable gas and the reactive gas to be introduced into the evacuated chamber through individual inlets or through a common inlet. The present invention is concerned with controlling the introduction of these gases such that the problems associated with the formation of undesirable target layers is obviated or at least minimised.